Chemical and physical analyses of a machine fluid provide information about the condition of the fluid as well as the wear status of the machine using the fluid. Thus, the machine fluid analysis has been widely used for determining lubricant conditions and wear mechanisms in many industries. Further, significant efforts have been made to analyze the condition of the fluid in real time. The real time fluid monitoring mechanism allows the lubricant to be used to its fullest potential with minimized machinery downtime, thereby resulting in increased savings and productivity.
As the oil degrades, the anti-oxidant additive package becomes depleted and the base oil oxidizes. A phenolic inhibitor, which is one of the most common anti-oxidant additives, works to neutralize the free radicals that cause oxidation. Further, aromatic amines, which are also commonly used anti-oxidant additives, work to trap the free radicals. Some anti-wear additives, such as zinc dialkyldithiophosphate (ZDDP), pull double duty as anti-oxidants. The anti-wear additives decompose peroxides formed as a by-product of oxidation and other chemical reactions. Over time, the anti-oxidant additives become depleted until they can no longer effectively protect the base oil.
Oxidation is the major process of the oil degradation, which is caused by the free radical reactions catalyzed by metals and accelerated by heat. The oil oxidation leads to increases in viscosity and acidity, as well as the formation of degradation products such as gum, varnish and sludge.
While the cost of the lubricant itself may be insignificant, the numerous factors for potential collateral damage to the machine are corrosion, varnishing, loss of lubricity, poor demulsibility, filter plugging, etc. Therefore, the oil quality analysis is necessary to increase the lifetime of the machine.
Typically, the lubricant oxidation has been estimated by laboratory detecting methods using a total acid number (TAN), a total base number (TBN) and a Fourier transform infrared (FTIR) analysis.
According to the ASTM (American Standard Test Method) D664, the TAN test employs a potassium hydroxide (KOH) reagent to neutralize acid in the oil. The volume of alkaline reagent required to reach the point of neutralization is a function of the concentration of acid in the oil. The premise is that when the oil oxidizes, organic acids are produced and collected in the oil, thus causing the TAN to rise. The TAN of an oil sample indicates the aging of the oil. Thus, it is typically used to determine when the oil must be changed.
The FTIR analysis effectively measures the concentration of various organic or metallo-organic materials, which are present in the oil. When the oil oxidizes, the hydrocarbon oil molecules become recompounded into soluble and insoluble oxidation by-products. The FTIR analysis measures the accumulation of these by-products at wave numbers of 1800˜1670 cm−1. However, the above techniques are unacceptable for an on-line oil monitoring system due to their complexity and high cost of equipment.
Various methods and devices for testing the oil quality in real time have been developed and introduced. For example, measuring a dielectric constant change in the lubricant is applied to an oil quality monitoring device. This is achieved by measuring the change in AC impedance, either by measuring the change in frequency when connected in a LC resonant circuit or by measuring the change in the dielectric loss tangent. U.S. Pat. No. 6,459,995 discloses a method and a device for measuring the oil quality based on the permittivity of the oil. The device comprises a capacitive sensor exposed to the oil and an oscillator circuit including a LC or crystal oscillator, which provides an output signal having amplitude dependent upon the loss tangent of the oil. That is, the output of the oscillator varies in response to the changes in the loss of the dielectric medium (oil), which in turn are determined principally by the changes in the oil acidity and polar oxidation products. The change in the amplitude of the oscillator output provides a measure of the oil quality. However, the above method based on the measurement of the dielectric constant is disadvantageous in that it is incapable of recognizing the main reason for the oil degradation—fuel content, water content, oil oxidation or particle contamination.
U.S. Pat. No. 5,789,665 discloses a method and a device for determining the deterioration of lubricating oil in real time based on the measurement of an electrical resistance. The device comprises a sensor employing a polymeric bead matrix layer between two permeable conducting surfaces, which measure the electrical properties of the polymeric matrix. The bead matrix contains a charged ion group that serves as a conducting medium for measuring the solvent properties of the oil. The device further comprises a housing for accommodating a conductive mesh containing small amounts (several milligrams) of ion-charged resin beads. The entire device is immersed in the oil so that the oil can enter into the housing. By using the device, the oil degradation is detected based on a correlation of a relative change in the electrical properties of the beads with a relative change in the solvent properties of the oil. In other words, as the oil changes from a nonpolar (clean) condition to a polar (oxidized contaminated) condition, the interaction between the charged ion groups and the ion-charged resin bead group is also changed. However, the above detecting method has the disadvantages of low reliability and complication of replacing the used polymer beads every time when the degraded oil is changed by new one.
As an alternative to the detecting method, which is based on the measurement of the electrical resistance, several optical techniques (particularly, a fluorescence analysis technique) were proposed to monitor the oil quality in real time. The main problem in applying the fluorescence analysis technique to the oil quality estimation is that there is an influence of high optical oil density on recording the signal of the fluorescence emission. This is because the oil color becomes darkened as the oil is used.
An oil quality monitoring method and a device capable of avoiding such problem is disclosed in U.S. Pat. No. 6,633,043. The method of this patent relates to a time-resolved and laser-induced fluorescence spectroscopy for the characterization and fingerprinting of the oil. This method uses the phenomenon wherein the shapes of the time-resolved fluorescence spectra of the fresh oil and the degraded oil vary in different manners. The method comprises the steps of: exposing an unknown oil sample to a pulse of ultraviolet laser radiation; measuring the intensity of the fluorescence over the spectrum of wavelengths of light of the oil sample at specific narrow time gates within the temporal response of the laser pulse to form a time-resolved spectrum; normalizing the time-resolved spectrum at a particular emission wavelength; plotting the time-resolved spectrum in contours as functions of wavelength and time; comparing the resultant plots of the plotting step with those of similar oil samples taken at known levels of degradation; and determining the condition of the unknown oil sample based on the similarity of the resultant plots with those of the particular known oil sample. The monitoring device comprises a pulsed UV laser for irradiating an oil sample contained in a quartz cuvette. The fluorescence signal of the oil sample is steered by quartz collecting lenses onto entrance slits of a medium-resolution of monochromator for dispersion and is then detected by a photo multiplier mounted at exit slits of the monochromator. The detected fluorescence signal is sent to a signal processor coupled with a gated integrator and is then sampled and digitized according to specific time gates and time gate widths by means of a computer. However, the device is rather complex for on-board realization and further needs expensive equipment.
U.S. Patent Application No. 20050088646 discloses another fluorescent technique taking into account of high optical density of oil. A change in oil fluorescence during the oil oxidation depends on the oil type (its base stock and additives). Although the oil fluorescence increases with the oxidation, the variation in the output signal of a sensor for detecting the fluorescence may be about zero since the increased oil fluorescence can be compensated by the oil absorption. To solve this problem, there is provided a fluorescence detector with two path-lengths, which provide two output signals corresponding to incident optical powers. Based on these two output signals, two parameters—fluorescence emission and optical density of test oil—are estimated simultaneously. However, this technique must use two path-lengths for fluorescence emission measurement and optical density measurement, thereby complicating the detection process and the associated device.
As discussed above, the above prior art devices and methods for monitoring oil oxidation have certain drawbacks and limitations in application.